Loop antennas are one of various kinds of antennas known in the art. A loop antenna is a type of antenna that is constructed by forming a conductor in the shape of a loop and that causes the conductor to operate as a coil. A method for adjusting the frequency characteristic of such a loop antenna is proposed in order to enable the loop antenna to transmit or receive a radio wave of a desired frequency (for example, refer to Japanese Laid-open Patent Publication No. 2001-160124 and International Publication WO 2012/137330).
For example, Japanese Laid-open Patent Publication No. 2001-160124 discloses a method which places, in the vicinity of an antenna forming a resonant circuit, a closed loop antenna or a conductor such as a metal plate or another resonant circuit in order to adjust the inductance of the antenna and thereby adjust the resonant frequency of the resonant circuit.
On the other hand, International Publication WO 2012/137330 proposes a metal sheet which is attached to a noncontact communication tag in order to maximize the communication distance at a desired frequency. The metal sheet includes a first metal part and a second metal part disposed at positions symmetrical about the center of the metal sheet, and is attached to the tag so that a portion of the first metal part and a portion of the second metal part respectively contact the tag.